Aluminum silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys, while alloys containing more than 11.6% silicon are referred to as hypereutectic alloys.
Hypoeutectic aluminum silicon alloys, those containing less than 11.6% silicon, have a microstructure consisting of primary aluminum dendrites with a eutectic composed of acicular silicon in an aluminum dendritic matrix.
Hypoeutectic aluminum silicon alloys often contain iron to prevent “sticking” of the alloy to steel casting molds, when such alloys are used in traditional die casting methods. In the microstructure of such alloys, the iron occurs as elongated needle-like structures.
The solidification range, which is a temperature range over which the alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider or greater the solidification range, the longer it will take an alloy to solidify at a given rate of cooling. During a hypoeutectic aluminum silicon alloy's descent through the solidification range, the aluminum dendrites are the first to form. As time elapses and the cooling process proceeds, the aluminum dendrites grow larger, eventually touch, and form a dendritic network. During this time frame, elongated iron needle-like structures also form and tend to clog the narrow passageways of the aluminum dendritic network, restricting the flow of eutectic liquid. Such phenomena tends to increase the instance of microporosity in the final cast structure. A high degree of microporosity is undesirable, particularly when the alloy is used for engine blocks, because high microporosity causes leakage under O-ring seals on machined head deck surfaces, and lowers the torque carrying capacity of machined threads. Further, hypoeutectic aluminum silicon alloy engine blocks are designed to have electro-deposited material, such as chromium, on the cylinder bore surfaces for wear resistance. However, the aforementioned microporosity prevents the adhesion of the electro-deposited chrome plating. Similarly, a hypereutectic aluminum silicon alloy, cast using a high pressure die casting method, also produces a porous structure in the parent bore material that contributes to high oil consumption.
Therefore, it would be advantageous to reduce the iron needle-like structure as well as the silicon eutectic particle size to facilitate interdendritic feeding and correspondingly reduce microporosity.
Hypoeutectic aluminum silicon alloys generally have poor ductility because of the large irregular shape of the acicular eutectic silicon phase. It has been recognized that the growth of the eutectic silicon phase can be modified by the addition of small amounts of sodium or strontium, thereby increasing the ductility of the hypoeutectic aluminum silicon alloy. Such modification further reduces microporosity as the smaller eutectic silicon phase structure facilitates interdendritic feeding.
U.S. Pat. No. 5,234,514 relates to a hypereutectic aluminum silicon alloy having refined primary silicon and a modified eutectic. The aforementioned patent is directed to modifying the primary silicon phase and the silicon phase of the eutectic through the addition of phosphorus and a grain refining substance. When this alloy is cooled from solid solution to a temperature beneath the liquidus temperature, the phosphorus acts in a conventional manner to precipitate aluminum phosphide particles, which serve as an active nucleant for primary silicon, thus producing smaller refined primary silicon particles having a size generally less than 30 microns. However, the '514 patent indicates that the same process could not be used with a hypereutectic aluminum silicon alloy modified with sodium or strontium, because the sodium and strontium neutralize the phosphorous effect, and the iron content of the alloy still causes precipitation of the iron phase that hinders interdendritic feeding.
U.S. Pat. No. 6,364,970 is directed to a hypoeutectic aluminum-silicon alloy. The alloy according to the '970 patent contains an iron content of up to 0.15% by weight and a strontium refinement of 30 to 300 ppm (0.003 to 0.03% by weight). This hypoeutectic alloy has a high fracture strength resulting from the refined eutectic silicon phase resulting from the addition of strontium to the alloy.
Hypereutectic aluminum silicon alloys have been used to produce engine blocks for outboard and stern drive motors in the recreation boating industry. Such alloys are advantageous for use in engine blocks as they provide a high tensile strength, high modulus, low coefficient of thermal expansion, and are resistant to wear.
High pressure die cast hypoeutectic aluminum silicon alloys have seen limited use for marine propellers as they are brittle and lack ductility. Due to their greater ductility, aluminum magnesium alloys are in general used for marine propellers. Aluminum magnesium alloys are advantageous as they provide high ductility and durability, but the repairability of such aluminum magnesium propellers is limited. The addition of magnesium to aluminum silicon alloys has been found to increase the ductility of propellers while providing an advantageous degree of durability. Still, it has been found that aluminum magnesium alloys are significantly more expensive to die cast into propellers because the casting temperature is significantly higher and because the scrap rate is much greater.
For cost and geometrical tolerance reasons, propellers for outboard and stern drive motors are traditionally cast using high pressure die cast processes. However, propellers may also be cast using a more expensive semi-solid metal (SSM) casting process. In the SSM process, an alloy is injected into a die at a suitable temperature in the semi-solid state, much the same way as in high pressure die casting. However, the viscosity is higher and the injection speed is much lower than in conventional pressure die casting, resulting in little or no turbulence during die filling. The reduction in turbulence creates a corresponding reduction in microporosity.
Regardless of how such propellers are cast, propellers regularly fracture large segments of the propeller blades when they collide with underwater objects during operation. This is due to the brittleness of traditional propeller alloys.
As a result, the damaged propeller blades cannot be repaired as the missing segments are lost at the bottom of the body of water in which the propeller was operated. Furthermore, the brittleness inherent in traditional aluminum-silicon alloys prevents efficient restructuring of the propellers through hammering. Thus, it is desirable to provide a propeller that only bends, but does not break upon impact with an underwater object.